1. Briefly explain the major contribution(s) that each of the following scientists made to our understanding of atomic theory. Include any relevant theories, models, and/or experiments in your answer.
Scientist Major Contribution(s)
Dalton Developed the atomic theory postulating that all elements are made up of tiny particles called atoms
Thomson Used cathode rays to discover the electron; Developed the Plum Pudding model of the atom (diffuse cloud of positive charge with negative electrons embedded within).
Rutherford Conducted the famous Gold Foil Experiment and discovered that atoms contain a tiny, dense positively charger nucleus. Later also discovered the proton.
Chadwick Continued the work of Rutherford and discovered the neutron.
Mendeleev Developed the first periodic table arranging elements according to their atomic weight in groups and families that exhibit similar physical and chemical properties
Bohr Studied hydrogen atoms and light emission to prove that electrons orbit the nucleus of an atom at certain, distinct radii related to their energy state. Heisenberg Used the Uncertainty Principle to explain that the momentum and position of
an electron cannot be determined with certainty and that electrons randomly move around the nucleus of an atom.
2. Answer the following questions about isotopes. a. What is an isotope?
Different atoms of the same element that have the same number of protons and electrons, but a different number of neutrons (mass).
b. Carbon-14 and nitrogen-14 both have the same mass yet they are different elements. Explain how they are similar and how they are different.
Both elements have 14 protons and neutrons. Carbon-14 has 6 protons, 8 neutrons, and 6 electrons while Nitrogen-14 has 7 protons, 7 neutrons, and 7 electrons. These are no isotopes; rather, they are elements with the same overall mass.
c. Magnesium has three naturally occurring isotopes: 24Mg (23.985 g/mol) with 78.99
% abundance; 26Mg (24.986 g/mol) with 10.00 % abundance; and Mg25 (25.937
g/mol) 11.01% abundance. Determine the average atomic mass of Mg. X = (23.985)(0.7899) + (24.986)(0.1000) + (25.937 )(0.1101)
X = 24.3 g/mol
3. What is an atom? What is an ion? Explain how they are similar and how they are different. Why do atoms become charged? What charges are likely for different groups of elements on the periodic table?
An atom is the building block, or basic unit of an element. When an atom gains or loses electrons it becomes an ion (when an atom gains electrons it becomes negatively charged and is referred to as an anion; when an atom loses electrons it becomes positively charged and is referred to as a cation).
4. Make a table and fill in the following information for each of the atoms or ions given below: Symbol, number of protons, number of neutrons, number of electrons, atomic mass, and charge.
Symbol Protons Neutrons Electrons Mass Charge
13 14 13 27 0
30 34 28 64 2+
82 125 82 207 0
17 18 18 35
1-92 146 92 238 0
47 61 46 108 1+
19 20 19 39 0
34 45 36 79
2-5. Answer the following questions about groups and patterns on the periodic table. a. What are some properties common to most metals?
Metals are typically good conductors of heat and electricity, are malleable and ductile, are lustrous and silver in appearance, and make positive charges (cations) when they react.
b. What are some properties common to most non-metals?
Nonmetals are typically brittle, poor conductors of heat and electricity, brightly colored, and make negative charges when they react.
c. What are semi-metals (metalloids)? Where are these groups found on the periodic table?
Semimetals are elements that have properties of both metals and nonmetals. They are found on the periodic table on the “stair-case” that separates metals and nonmetals.
6. Define atomic radius, ionization energy, electron affinity, and electronegativity.
Atomic radius is the approximate distance from the nucleus of an atom to its outermost (valence) electrons.
Ionization energy is the energy required to remove the outermost electron of an atom.
Electron affinity is the change in energy when a gaseous atom gains an electron.
Electronegativity is the ability of an atom in a gaseous state to remove an electron from another atom.
7. Explain the following observations
a. The atomic radius of Cl is less than that of P.
C,l is further right on the P.T. and has a greater nuclear attraction due to the greater number of protons in the nucleus.
b. The ionization energy of C is much greater than that of Si.
The first ionization of carbon is greater than that of silicon because silicon has an additional electron level, which means there is more electron shielding in silicon that lowers the effective nuclear charge of the atom making it easier to remove the valence electron
c. The atomic radius of Mg is more than that of O.
Magnesium is larger than neon because it has an additional level of electrons. This increases the electron shielding in the atom and decreases the effective nuclear charge, increasing the atomic radius.
d. The ionization of K is much smaller than the ionization energy of Ca.
The ionization energy is smaller for potassium than calcium because calcium has more protons in its nucleus. This increases the effective nuclear charge making it harder to remove the outermost electron.
8. Write a complete electron configuration for the following elements. Element Electron configuration
Sodium 1s22s22p63s1
Neon 1s22s22p6
Cobalt 1s22s22p63s23p64s23d7 Strontium 1s22s22p63s23p64s23d104p65s2
Tin 1s22s22p63s23p64s23d104p65s24d105p2
Uranium 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p67s26d15f3
9. What is the frequency of a microwave with a wavelength of 433 nm? ν = (3.00 x 108 m/s)/(433 x 10-9 m) = 6.93 x 1014 Hz
10. What color is visible light with a frequency of 5.25 x 1014 Hz?
λ = (3.00 x 108 m/s)/(5.25 x 1014 Hz) = 5.71 x 10-7 m = 571 nm; Visible light (yellow)
11. The MRI body scanner used in hospitals operates with 400,000,000 Hz radiofrequency energy. How much energy does this correspond to for a single photon and for a mole of photons?
Ephoton = h ν = (6.626 x 10-34 J s)(400 x 106 Hz) = 2.65 x 10-25 J Emole = (2.65 x 10-34 J)(6.02 x 1023) = 0.160 J
12. Explain what a mole is.
A mole represents the amount of substance in chemistry. Just like the word dozen
represents 12 units, or the word couple represents 2 units, the word mole represents 6.02 x 1023 units.
13. Using the data below, Draw the electron transitions that occur in the visible light spectrum, 700-750nm
Energy level 1 2 3 4 5 6
ionization 2371kJ 593kJ 263kJ 148kJ 94.5kJ 65.6kJ
n=6 n=5 n=4 n=3 n=2
n=1 change in Energy (kJ) 2305.4 2276.5 2223 2108 1778
wavelength (nm) 51.89 52.55 53.81 56.75 67.28
n=2 change in Energy (kJ) 527.4 498.5 445 330
wavelength (nm) 226.82 239.97 268.82 362.50
n=3 change in Energy (kJ) 197.4 168.5 115
wavelength (nm) 606.01 709.95 1040.23
| Chemistry 5
